Lubricating oil base oil, lubricating oil composition, and method for using lubricating oil composition

ABSTRACT

A lubricating oil base oil may include a solid surface distance of 1.0 nm or more at a surface pressure of 12.4 MPa and a normalized peak intensity ratio of more than 0.2 to 1.0 measured using a resonance shear measurement system and a lubricating oil composition may include the lubricating oil base oil.

TECHNICAL FIELD

The present invention relates to a lubricating oil base oil, a lubricating oil composition comprising the lubricating oil base oil, and a method for using a lubricating oil composition.

BACKGROUND ART

In recent years, efforts have been made in each industrial field for the purpose of improving fuel efficiency, and as a means for improving fuel efficiency, continuously variable transmissions (CVTs) are often used in power transmission devices.

CVTs are transmission devices that can continuously change the output rotation with respect to a constant input rotation, and use a mechanism such as a friction drive that transmits power using a metal belt or chain or a traction drive that does not use such an element.

In particular, a traction drive is compact and capable of transmitting a large amount of power, and is being put to practical use in the field of automobiles.

Then, a lubricating oil used for a traction drive is required to have a high traction coefficient from the viewpoint of improving power transmission.

For example, Patent Literature 1 discloses, as a lubricating oil base oil composition with an improved flash point and low temperature fluidity without compromising a high temperature traction coefficient, a lubricating oil base oil composition consisting of a naphthenic synthetic lubricating oil base oil having an ignition point of 180° C. or lower and a paraffiniic synthetic lubricating oil base oil such as polya-olefin having a higher ignition point than the naphthenic synthetic lubricating oil base oil.

Citation List Patent Literature

Patent Literature 1: JP 2000-204386 A

SUMMARY OF INVENTION Technical Problem

The lubricating oil base oil composition described in Patent Literature 1 can also be suitably used for lubricating a traction drive, but there is a demand for a lubricating oil composition that can be more suitably used for lubricating a traction drive.

Solution to Problem

The present invention provides a lubricating oil base oil which is prepared such that the solid surface distance and the normalized peak intensity ratio at a surface pressure of 12.4 MPa measured using a resonance shear measurement system are each within a predetermined range.

More specifically, the invention provides the following [1] to [10].

[1] A lubricating oil base oil having a solid surface distance of 1.0 nm or more measured using a resonance shear measurement system at a surface pressure of 12.4 MPa and having a normalized peak intensity ratio of more than 0.2 to 1.0 at the solid surface distance. [2] The lubricating oil base oil according to [1], wherein a viscosity _(rlp) of the lubricating oil base oil at 40° C. measured at a surface pressure of 12.4 MPa is 10 to 50 mPa·s. [3] The lubricating oil base oil according to [1] or [2], wherein a viscosity ηP of the lubricating oil base oil at 40° C. measured at normal pressure is 5 to 50 mPa·s. [4] The lubricating oil base oil according to any one of [1] to [3], wherein the lubricating oil base oil comprises one or more selected from an ester synthetic oil, a naphthenic synthetic oil, and a mineral oil. [5] The lubricating oil base oil according to any one of [1] to [4], wherein the lubricating oil base oil is used in a lubricating oil composition for a traction drive. [6] A lubricating oil composition comprising the lubricating oil base oil according to any one of [1] to [5]. [7] The lubricating oil composition according to [6], wherein the lubricating oil composition is used for lubricating a traction drive. [8] The lubricating oil composition according to [7], wherein the lubricating oil composition is used for lubrication in a nanospace in which a distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm. [9] A method for using a lubricating oil composition, comprising using the lubricating oil composition according to [6] for lubricating a traction drive. [10] The method for using a lubricating oil composition according to [9], wherein the method is used for lubrication in a nanospace in which a distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm.

Advantageous Effects of Invention

The use of the lubricating oil base oil of the present invention enables the preparation of a lubricating oil composition that can be used more preferably for lubricating a traction drive.

DESCRIPTION OF EMBODIMENTS [Lubricating Oil Base Oil]

The lubricating oil base oil of the present invention is prepared such that it has a solid surface distance of 1.0 nm or more measured using a resonance shear measurement system at a surface pressure of 12.4 MPa and a normalized peak intensity ratio of more than 0.2 to 1.0 at the solid surface distance.

The “resonance shear measurement system” functions as a surface forces apparatus by which a liquid is sandwiched between two solid surfaces facing each other, and the surface force (repulsive force/attractive force) generated in the direction perpendicular to the two solid surfaces facing each other is measured at the same time as the distance between the surfaces. In addition, the system also functions to evaluate the properties of the liquid sandwiched between the surfaces (structural behavior, viscosity, friction, lubrication, shear resistance) by resonance shear response.

An example of such “resonance shear measurement system” is “RSM-1” (trade name; manufactured by ADVANCE RIKO, Inc.)

It is known that when the distance between surfaces is reduced to nanometers, the liquid sandwiched between the solid surfaces causes the formation of an ordered structure, a dramatic increase in viscosity, and the like due to the effects of confinement and interface, and thus it exhibits properties that are significantly different from those in the bulk liquid form.

Various lubricating base oils are sandwiched between two opposing solid surfaces, the solid surface distance (distance between the solid surfaces) was reduced to nanometers with a shift from a state in which the two solid surfaces faced each other via air (AS: separation in air) to a state in which the two solid surfaces facing each other came into contact with each other (SC: solid contact), thereby measuring the change in the surface pressure value.

As a result, in the vicinity of the AS side, even when the solid surface distance was gradually reduced, the surface pressure increased at a constant rate for a while; however, in the process of trying to reduce the solid surface distance to 1 nm, rapid thickening was induced, and thus the presence of a lubricating oil base oil with a significant increase in the surface pressure was confirmed.

It was also found that the value of the solid surface distance when the surface pressure rises significantly differs depending on the type of structure forming the lubricating oil base oil, which is a sample oil.

In the process of reducing the solid surface distance to 1 nm in this way, when a significant increase in the surface pressure value is observed, it is thought that the lubricating oil base oil forms and retains a strong oil film due to rapid thickening. On top of this, the oil film has a thickness of at least 1 nm.

The properties of the lubricating oil base oil capable of forming such an oil film can be suitably applied for the purpose of lubricating various apparatuses, in particular, traction drives.

In addition, the term “traction drive” used herein means a power transmission mechanism which transmits torque through an oil film under elastic fluid lubrication.

Two properties are required for the oil film formed by a traction drive, which are: abrasion resistance that can prevent solid surface contact between two opposing surfaces which roll and come into contact with each other and can suppress the wear of the solid surface; and a high traction coefficient for improving driving force.

Since the oil film formed in the above process has a thickness of 1 nm or more, it is considered that the oil film has good retention and can exhibit excellent abrasion resistance. Further, since the oil film, which has been formed and retained by thickening to such an extent that the surface pressure value is remarkably increased, is strong, it is considered that a high traction coefficient is achieved and the power transmission is improved. Therefore, it can be said that a lubricating oil composition using such a lubricating oil base oil is suitable for the purpose of lubricating a traction drive.

The lubricating oil base oil of the present invention was prepared with a focus on the solid surface distance at a surface pressure of 12.4 MPa and the normalized peak intensity ratio at the solid surface distance.

The “solid surface distance at a surface pressure of 12.4 MPa” is specified as a solid surface distance when in the process of trying to reduce the solid surface distance to 1 nm, rapid thickening is induced, and a significant increase in the surface pressure is observed.

After rapid thickening of the lubricating oil base oil is induced and the surface pressure reached 12.4 MPa, a state in which surface pressure is applied to make it difficult to reduce the solid surface distance is achieved.

The lubricating oil base oil of the present invention is prepared such that the solid surface distance is 1 nm or more at a surface pressure of 12.4 MPa.

In a case in which a lubricating oil composition comprising such a lubricating oil base oil is used for lubrication in a nanospace in which the distance between two opposing surfaces is several nanometers, it is considered that a strong oil film having a thickness of 1 nm or more is formed such that the oil film can be retained and the lubrication property can be improved. In particular, in a case in which the lubricating oil composition is used for lubricating a traction drive, since the thickness of the oil film is 1 nm or more, the retention property is favorable, and excellent abrasion resistance can be exhibited, which can prevent solid surface contact between two opposing solid surfaces which roll and come into contact with each other and can suppress the wear on the solid surface.

Note that a lubricating oil base oil with which the solid surface distance with a surface pressure reaching 12.4 MPa is less than 1 mm or a lubricating oil base with which the surface pressure cannot reach 12.4 MPa even with a reduction of the solid surface distance to around the vicinity of the SC side has a problem in oil film retention and is difficult to use for the above-described purpose of lubrication in the nanospace.

From the above perspective, the solid surface distance for the lubricating oil base oil at a surface pressure of 12.4 MPa in one aspect of the present invention is preferably 1.2 nm or more, more preferably 1.5 nm or more, still more preferably 2.0 nm or more, even more preferably 2.5 nm or more, and usually 10 nm or less, but preferably 9 nm or less, more preferably 8 nm or less, and particularly preferably 7 nm or less.

Further, the lubricating oil base oil of the present invention is prepared such that the normalized peak intensity ratio at the solid surface distance at a surface pressure of 12.4 MPa is more than 0.2 to 1.0.

The term “normalized peak intensity ratio” used herein means the ratio of the peak intensity I of the resonance frequency at a surface pressure of 12.4 MPa to the peak intensity I_(sc) of the resonance frequency in a state in which two opposing solid surfaces are in contact with each other (SC) [I/I_(sc)].

The value of the normalized peak intensity ratio is an index of the hardness of an oil film formed in the nanospace.

In other words, it is considered that the lubricating oil base oil having a normalized peak intensity ratio of more than 0.2 retains a strong oil film in the nanospace. It is considered that the property of retaining a strong oil film achieves a high traction coefficient and contributes to the improvement of power transmission in a case in which a lubricating oil composition comprising the lubricating oil base oil is used for lubricating a traction drive.

Meanwhile, a lubricating oil base oil having a normalized peak intensity ratio of 0.2 or less is considered to have insufficient hardness of the oil film. It can be said that when it is used for lubricating a traction drive, the traction coefficient becomes low, which is problematic in terms of power transmission.

From the above viewpoint, the normalized peak intensity ratio at a surface pressure of 12.4 MPa of the lubricating oil base oil in one aspect of the present invention is preferably 0.22 to 1.00, more preferably 0.25 to 1.00, still more preferably 0.35 to 1.00, even more preferably 0.50 to 1.00, and particularly preferably 0.70 to 1.00.

The solid surface distances and the normalized peak intensity ratio at a surface pressure of 12.4 MPa of the lubricating oil base oil described herein can be measured by a resonance shear measurement system (e.g., trade name: “RSM-1” (manufactured by ADVANCE RIKO, Inc.). As specific measurement procedures using the resonance shear measurement system, the method described in Reference 1 (A new physical model for resonance shear measurement of confined liquids between solid surfaces, REVIEW OF SCIENTIFIC INSTRUMENTS 79, 113705 2008) can be mentioned, and more specifically, the measurement can be performed based on the method described in the Examples.

In view of such properties, the lubricating oil base oil in one aspect of the present invention is preferably used in a lubricating oil composition for a traction drive.

For the lubricating oil base oil in one aspect of the present invention, from the viewpoint of obtaining a lubricating oil base oil that can be prepared into a lubricating oil composition suitable for lubricating a traction drive, the viscosity η_(P) at 40° C. measured at a surface pressure of 12.4 MPa is preferably 10 to 50 mPa·s, more preferably 12 to 45 mPa·s, still more preferably 14 to 40 mPa·s, and even more preferably 16 to 35 mPa·s.

In addition, for the lubricating oil base oil in one aspect of the present invention, from the viewpoint of obtaining a lubricating oil base oil that can be prepared into a lubricating oil composition suitable for lubricating a traction drive, the viscosity η₀ at 40° C. measured at normal pressure (0.10 MPa) is preferably 5 to 50 mPa·s, more preferably 7 to 45 mPa·s, and still more preferably 9 to 40 mPa·s.

For the lubricating oil base oil in one aspect of the present invention, the ratio [η_(P)/η₀] of the viscosity η_(P) at 40° C. measured at a surface pressure of 12.4 MPa and the viscosity η₀ at 40° C. measured at normal pressure (0.10 MPa) is preferably 1.05 to 2.0, more preferably 1.1 to 1.7, and still more preferably 1.15 to 1.5.

As specified herein, the viscosity η₀ can be measured according to the method described in Reference 2 below, and the viscosity Tip can be measured using a rolling-ball high-pressure viscometer according to the method described in Reference 3 below. More specific measurement procedures are as described in the Examples below.

Reference 2: API Standard 2540 (1980) ‘Manual of Measurement Standard Chapter11.1—Lolume Correction Factors, Volume XIV’ Appendix A

Reference 3: Tribologist, vol. 55, No.9 (2010), pp. 41-52 (published by Idemitsu Kosan Co., Ltd.)

The kinetic viscosity of the lubricating oil base oil in one embodiment of the present invention at 100° C. is preferably 2.0 to 10.0 mm²/s, more preferably 2.2 to 8.5 mm²/s, still more preferably 2.5 to 7.0 mm²/s, and even more preferably 2.8 to 6.5 mm²/s.

In addition, the viscosity index of the lubricating oil base oil in one aspect of the present invention is usually 45 or more, but from the viewpoint of obtaining a lubricating oil base oil having a small temperature dependency, it is preferably 70 or more, more preferably 80 or more, and still more preferably 90 or more.

The kinetic viscosity and the viscosity index of the lubricating oil base oil described herein mean the values measured and calculated in accordance with JIS K2283:2000.

The lubricating oil base oil in one aspect of the present invention can be composed of one or more selected from mineral oils and synthetic oils. From the viewpoint of preparing the lubricating oil base oil such that the values of the solid surface distance and the normalized peak intensity ratio are within the above ranges, the lubricating oil base oil comprises preferably one or more selected from an ester synthetic oil, a naphthenic synthetic oil, and a mineral oil, more preferably one or more selected from an ester synthetic oil and a naphthenic synthetic oil, and still more preferably an ester synthetic oil.

As stated above, the values of the solid surface distance and the normalized peak intensity ratio are likely to change depending on the molecular structure of the lubricating oil base oil. Therefore, for example, even in the case of an ester synthetic oil, the values of the solid surface distance and the normalized peak intensity ratio change depending on the difference in the molecular structure. Further, in the case of a mixture composed of two or more different compounds, these values also change depending on the content ratio of each compound.

For the lubricating oil base oil of the present invention, by selecting a compound having a specific molecular structure and adjusting the content ratio of each compound, the values of the solid surface distance and the normalized peak intensity ratio are set to be within the above ranges.

For example, it is preferable for an ester synthetic oil to comprise an ester compound of aliphatic polyvalent alcohol from the viewpoint of adjusting the solid surface distance and the normalized peak intensity ratio to large values. The valence of aliphatic polyvalent alcohol is preferably 2 to 6.

In addition, the ester compound includes preferably a compound having an alkyl group having 6 to 20 carbon atoms, and more preferably a compound having an alkyl group having 8 to 12 carbon atoms.

Further, the ester synthetic oil is composed of preferably at least one compound having an alkyl group having 8 to 12 carbon atoms, and more preferably a mixture composed of at least two or more different compounds having an alkyl group having 8 to 12 carbon atoms.

The naphthenic synthetic oil comprises preferably a naphthenic compound having 10 to 25 carbon atoms, and more preferably a naphthenic compound having 12 to 20 carbon atoms from the viewpoint of adjusting the solid surface distance and the normalized peak intensity ratio to large values.

The naphthenic compound includes preferably A compound having at least one ring selected from a bicyclo[2.2.1]heptane ring, a bicyclo[2.2.2]octane ring, and a bicyclo[3.3.0]octane ring, and more preferably a dimer having two of these rings.

In addition, the naphthenic compound includes preferably a compound having an alkyl group having 1 to 3 carbon atoms, and more preferably a compound having a plurality of alkyl groups having 1 to 3 carbon atoms.

Examples of mineral oils include: atmospheric pressure residual oils obtained by distillation of paraffinic crude oil, intermediate-base crude oil, naphthenic crude oil, and the like; distillates obtained by distilling these atmospheric pressure residual oils under reduced pressure; refined oils obtained by treating these distillates via one or more purification treatments such as solvent deasphalting, solvent extraction, hydrogenolysis, solvent dewaxing, catalytic dewaxing, and hydrorefining; and mineral oils (GTL) obtained by isomerizing wax (gas-to-liquid (GTL) wax) produced from natural gas by the Fischer-Tropsch method or the like.

Of these, the mineral oil is preferably a mineral oil composed of a compound having a hydrocarbon group having 12 to 50 carbon atoms, and more preferably a paraffinic mineral oil composed of a compound having a hydrocarbon group having 12 to 50 carbon atoms from the viewpoint of adjusting the solid surface distance and the normalized peak intensity ratio to large values. In addition, the number of carbon atoms of the hydrocarbon group is preferably 12 to 50, more preferably 16 to 50, still more preferably 20 to 50, and even more preferably 22 to 50 from the viewpoint of adjusting the solid surface distance and the normalized peak intensity ratio to large values.

The lubricating oil base oil in one aspect of the present invention may be a mixed base oil comprising a plurality of types of base oils. In the case of a mixed base oil, the blending ratio of each base oil may be adjusted such that the values of the solid surface distance and the normalized peak intensity ratio of the mixed base oil are within the above ranges.

The lubricating oil base oil in one aspect of the present invention preferably comprises an olefin synthetic oil such as polya-olefin as little as possible from the viewpoint of adjusting the solid surface distance and the normalized peak intensity ratio within the above ranges.

In view of the above, the content of an olefin synthetic oil in the lubricating oil base oil in one aspect of the present invention based on the total amount (100% by mass) of the lubricating oil base oil is preferably less than 10% by mass, more preferably less than 5% by mass, still more preferably less than 1% by mass, even more preferably less than 0.1% by mass, and particularly preferably less than 0.01% by mass.

The content of polya-olefin is also preferably in the above range.

[Lubricating Oil Composition]

The lubricating oil composition of the present invention comprises the above-described lubricating oil base oil of the present invention.

In the lubricating oil composition in one aspect of the present invention, the content of the lubricating oil base oil based on the total amount (100% by mass) of the lubricating oil composition is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, and even more preferably 85% by mass or more.

The lubricating oil composition in one aspect of the present invention may comprise various additives, if necessary.

Examples of such additives include antioxidants, detergent dispersants, pour point depressants, viscosity index improvers, anti-wear agents, extreme pressure agents, anti-foam agents, rust inhibitors, and corrosion inhibitors.

These additives for lubricating oil may be used singly or in combination of two or more kinds thereof.

The content of each of these additives can be adjusted as appropriate depending on type of the additive. The content of each additive is usually 0.001% to 15% by mass, preferably 0.005% to 10% by mass, and more preferably 0.01% to 5% by mass independently based on the total amount (100% by mass) of the lubricating oil composition.

[Applications of Lubricating Oil Composition]

Applications of the lubricating oil composition in one aspect of the present invention include traction drive fluid, transmission oil, hydraulic actuation oil, compressor oil, electrically insulating oil, and the like.

As described above, the lubricating oil base oil of the present invention is considered to have excellent abrasion resistance and high power transmission efficiency because a strong oil film can be formed and retained in the nanospace. Therefore, it is considered that the lubricating oil composition of the present invention comprising the lubricating oil base oil can exhibit the same properties.

Accordingly, the lubricating oil composition in one aspect of the present invention is preferably used as traction drive fluid for lubricating a traction drive, and it is more preferably used for lubrication in a nanospace in which the distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm.

The distance between two surfaces of two opposing solids constituting the traction drive corresponds to the solid surface distance in the resonance shear measurement system described above.

In addition, given the above-described properties of the lubricating oil composition of the present invention, the invention may also provide the following [1] and [2].

[1] A traction drive using the above-described lubricating oil composition in one aspect of the present invention. [2] A method for using a lubricating oil composition, the method comprising using the lubricating oil composition in one aspect of the present invention for lubricating a traction drive.

In [1] and [2] above, the lubricating oil composition in one aspect of the present invention is preferably used for lubrication in a nanospace in which the distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm.

EXAMPLES

Next, the present invention will be described in more detail with reference to the Examples below, but the present invention is not limited to these examples. The methods of measuring and evaluating various physical properties are as follows:

(1) Measurement and Calculation of Solid Surface Distance and Normalized Peak Intensity Ratio

The change in the surface pressure applied to a sample oil with the change in the solid surface distance was measured using a resonance shear measurement system (trade name: RSM-1; ADVANCE RIKO, Inc.).

Specifically, based on the method described in Reference 1 mentioned below, about 200 μl of any one of sample oils (α1) to (α7) was added dropwise between two surfaces of clean and smooth mica cleavage planes facing each other and brought in direct contact in a perpendicular cylinder mode using the resonance shear measurement system at room temperature (25° C.). The solid surface distance was calculated by the fringes of equal chromatic order (FECO) method. The solid surface distance was measured when the surface pressure reached 12.4 MPa by gradually reducing the solid surface distance with a shift from a state in which the two solid surfaces faced each other via air (AS) to a state in which the two solid surfaces facing each other came into contact with each other (SC).

In addition, for the resonance shear response, a voltage Uin was applied to a resonance shear unit so as to generate horizontal vibration, and the amplitude of vibration was measured by a capacitance meter to determine a voltage Uout, thereby evaluating the measured value as a vibration intensity (Uout/Uin). By analyzing the resulting resonant response (vibration intensity [Uout/Uin]) using the method described in Reference 1 mentioned below, the peak intensity I of the resonance frequency when the surface pressure reached 12.4 MPa and the peak intensity I_(SC) of the resonance frequency at SC were calculated, and the ratio of the two [I/I_(SC)] was designated as the normalized peak intensity ratio.

Reference 1: A new physical model for resonance shear measurement of confined liquids between solid surfaces, REVIEW OF SCIENTIFIC INSTRUMENTS 79, 113705 2008

(2) Viscosity η₀ at 40° C. at Normal Pressure (0.10 MPa)

Calculation was made in accordance with the method described in Reference 2 mentioned below.

The density (dt) at 40° C. used in the calculation of viscosity η₀ can be calculated by the following formula:

dt=d15×exp (−D×Δt×(1+0.8×D×Δt))

(in the formula, D=0.6278/1000×d15 (note that d15 is the density of the sample oil at 15° C. measured in accordance with JIS K2249-4: 2011).

Reference 2: API Standard 2540 (1980) ‘Manual of Measurement Standard Chapter11.1—Lolume Correction Factors, Volume XIV’ Appendix A (3) Viscosity η_(P) at 40° C. at Surface Pressure of 12.4 MPa

The viscosity η_(P) of the sample oil at a pressure of 12.4 MPa was calculated using a rolling-ball high-pressure viscometer in accordance with the method described in Reference 3 mentioned below.

Reference 3: Tribologist, vol. 55, No. 9 (2010), pp. 41-52 (published by Idemitsu Kosan Co., Ltd.)

(4) Kinetic Viscosity, Viscosity Index

Kinetic viscosity and viscosity index were measured and calculated in accordance with JIS K2283:2000.

Examples 1 to 4, Comparative Examples 1 to 3

Various physical properties, including the solid surface distance and the normalized peak intensity ratio, were measured for the following sample oils (α1) to (α7) obtained by a predetermined purification or synthesis method. The various physical properties measured are shown in Table 1.

Sample oil (α1): Ester synthetic oil comprising a mixture of ester compounds comprising a trimethylolpropane triester having an alkyl group having 8, 10, or 12 carbon atoms. Sample oil (α2): Naphthenic synthetic oil comprising an alicyclic polycyclic compound represented by the following formula.

Sample oil (α3): Paraffinic mineral oil comprising a compound having a hydrocarbon group having 22 to 50 carbon atoms, which has a dynamic viscosity at 100° C.=6.0 mm²/s. Sample oil (α4): Paraffinic mineral oil comprising a compound having a hydrocarbon group having 16 to 30 carbon atoms, which has a dynamic viscosity at 100° C.=3.1 mm²/s. Sample oil (α5): Polya-olefin, which has a dynamic viscosity at 100° C.=5.9 mm²/s. Sample oil (α6): Polya-olefin, which has a dynamic viscosity at 100° C.=3.9 mm²/s. Sample oil (α7): Polya-olefin, which has a dynamic viscosity at 100° C.=1.8 mm²/s.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 1 Ex. 2 Ex. 3 Sample Sample Sample Sample Sample Sample Sample oil (α1) oil (α2) oil (α3) oil (α4) oil (α5) oil (α6) oil (α7) Solid surface distance [nm] measured at 4.4 4.4 6.5 1.7 3.4 0.3 0.8 25° C., 12.4 MPa Normalized peak intensity ratio [−] 0.81 0.41 0.26 0.22 0.19 0.13 0.11 measured at 25° C., 12.4 MPa Viscosity η₀ [mPa · s] at 40° C., normal 18.2 20.6 29.0 10.1 24.4 14.1 3.9 pressure (0.10 MPa) Viscosity η_(P) [mPa · s] at 40° C., surface 22.1 29.0 39.8 12.6 30.4 17.5 4.7 pressure (12.4 MPa) Viscosity ratio η_(P)/η₀ 1.2 1.4 1.4 1.3 1.2 1.2 1.2 Kinetic viscosity [mm²/s] at 100° C. 4.3 4.0 6.0 3.1 5.9 3.9 1.8 Viscosity index 139 51 121 109 132 120 —

From the values of the solid surface distance and the normalized peak intensity ratio, it is considered that the sample oils (α1) to (α4) of Examples 1 to 4 can form and retain a strong oil film in the nanospace. Therefore, it can be said that these sample oils can be suitably used for lubricating a traction drive.

Meanwhile, since any of the sample oils (α5) to (α7) of Comparative Examples 1 to 3 had a low value of the normalized peak intensity ratio, it is considered that there is a problem in terms of the hardness of the formed oil film, and thus, it is presumed that any of them is not suitable for lubricating a traction drive. Further, since each of the sample oils (α6) and (α7) of Comparative Examples 2 and 3 had a solid surface distance of less than 1.0 nm, it is considered that the oil film retention is inferior in the nanospace. 

1. A lubricating oil base oil, having a solid surface distance of 1.0 nm or more, measured using a resonance shear measurement system at a surface pressure of 12.4 MPa, and having a normalized peak intensity ratio of more than 0.2 to 1.0.
 2. The base oil of claim 1, wherein a viscosity η_(P) of the base oil at 40° C. measured at a surface pressure of 12.4 MPa, is in a range of from 10 to 50 mPa·s.
 3. The base oil to claim 1, wherein a viscosity η_(P) of the base oil at 40° C., measured at normal pressure, is in a range of from 5 to 50 mPa·s.
 4. The base oil of claim 1, which comprises an ester synthetic oil, a naphthenic synthetic oil, and/or a mineral oil.
 5. A traction drive, comprising the base oil of claim
 1. 6. A lubricating oil composition,. comprising the base oil of claim
 1. 7. A traction drive, comprising the composition of claim
 6. 8. The traction drive of claim 7, wherein the composition lubricates in a nanospace in which a distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm.
 9. A method of lubricating a traction drive, comprising contacting the traction drive with the composition of claim
 6. 10. The method of claim 9, comprising lubricating in a nanospace in which a distance between two surfaces of two opposing solids constituting the traction drive is 1.0 to 10 nm. 